1. Field of the Invention
This invention relates to the control of acid mine drainage and acid rock drainage, and more specifically, to the treatment of sulfide bearing material with alkaline compounds combined with adjuvant surfactants and water.
2. Related Art
For years it has been known that mining or storage of sulfide bearing material (metal ores, coal seams, mine spoils, tailings, refuse, etc.) results in adverse environmental conditions, in particular, the generation of acidic waters. Acidic waters adversely affect flora and fauna and are a major environmental concern of producers, processors, regulatory agencies, and people who handle and store sulfide bearing material.
The oxidation of the mineral pyrite (FeS.sub.2) and other sulfur bearing compounds produce acidic waters. The chemical reactions involve the oxidation of sulfur bearing compounds in the presence of oxygen and water forming iron hydroxide and sulfuric acid. The acidic waters are commonly called Acid Mine Drainage (AMD) or Acid Rock Drainage (ARD).
The bio-activity of oxidizing bacteria (Ferrobacillus and Thiobacillus) greatly increases the rate of pyrite oxidation. These bacteria require an acidic environment in which to survive. Because the rate of pyritic oxidation is greater in the presence of these bacterium, the rate of sulfuric acid generation also increases. Once the acid generation process is initiated, these bacteria multiply, sustaining and escalating the pyrite oxidation rate and the associated rate of sulfuric add generation. It is not uncommon to have AMD and ARD water with very low pH values of 2.1 to 3.0.
Convention methods for treating sulfide bearing material chemically treat acidic water after it has been produced, but prior to the water leaving the mining or storage area. Therefore, conventional methods treat the by-product of a sulfide bearing material, but does not directly treat the source or sulfide bearing material itself. Various alkaline compounds are used for treating acidic water, including sodium hydroxide, anhydrous ammonia, limestone, phosphate, and calcium hydroxide. These chemicals are effective in varying degrees for neutralizing acidic waters, but they are not completely effective.
One disadvantage to the conventional method pertains to the flow of the acidic water. As the flow of acidic water from the mining or storage operation varies, the application rate of the neutralizing agent must also vary; otherwise, either over or under treatment of the acidic water occurs. In either case, the water discharged is not within the required quality specification and the environment is negatively affected.
Another disadvantage is that the acidic water dissolves and mobilizes several metals, in particular, iron and manganese, which under neutral or basic conditions remain relatively insoluble. However, the metals are at elevated levels in acidic water, therefore, direct discharge is not permitted. As a result of these elevated levels, these elements must be monitored and maintained within the allowable limits as determined by the National Pollution Discharge Elimination on System (NPDES) permitting process.
A further disadvantage requires continual treatment of the acidic waters to stay within the NPDES compliance limits. During the neutralization process of raising the pH of the acidic water, iron and manganese are precipitated out of solution forming a solid which slowly settles to the bottom of the treatment basin. These chemical precipitates gradually fill the treatment basin thereby decreasing the remaining liquid volume of the basin. The decreased liquid volume reduces the retention time of the water in the treatment basin, thereby reducing the time available for solids to settle out of the water. This situation increases the risk of discharging water which is not in compliance with NPDES criteria for pH, metals, and suspended solids.
A still further disadvantage is that acidic water migrates to the lowest point in a cool system. If movement of the acidic water is not contained within an impermeable barrier (e.g. pipe, plastic, glass, etc.) the acidic waters flows into the underlying groundwater system. Once the acidic water enters the groundwater system, the detrimental affects on flora and fauna becomes widespread. Containment and treatment of the affected groundwater system is difficult, if not impossible.
A still further disadvantage of the conventional method is that the generation of acidic waters is a long term situation that, with present treatment methods, requires perpetual chemical treatment. Therefore, funding for long term treatment of acidic waters and long term responsibility and liability for environmental protection is an extreme problem. The problem is further complicated by the fact that mining and sulfide handling operators (individuals and companies) are often not perpetual entities. As a result, there are many examples of operators abandoning properties that discharge acidic water. To date, there is no known solution to this problem, rather the issue is now addressed through regulatory actions or litigation dealing with responsibility and liability for proper treatment and costs, required maintenance and monitoring personnel, cleaning and disposal of treatment pond sediments, and methodology, cost and maintenance of disposal areas. The focus of such regulatory and/or civil actions is restricted, however, by the limitation of the current methodologies for treating acidic waters (i.e. treatment of the symptom and not the source).
Therefore, there is a need for a system and method of treating acidic generating sulfide bearing material prior to acidic water being produced by the water's exposure to the sulfide bearing material.